Glass-ceramic bodies have been employed extensively in the manufacture of culinary ware. Such materials have been utilized in that application both in the opaque state and in the transparent state. U.S. Pat. No. 4,093,468 describes one family of essentially transparent glass-ceramic compositions demonstrating chemical and physical properties suitable for use as culinary ware.
That patent discloses the production of glass-ceramic articles having base compositions within the Li.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 system, but which also contains TiO.sub.2 as a nucleating agent and Fe.sub.2 O.sub.3 as an impurity customarily present in the batch materials. The combination of TiO.sub.2 and Fe.sub.2 O.sub.3 causes the development of a yellowish coloration in the glass-ceramic products. The patent teaches the addition of neodymium oxide (Nd.sub.2 O.sub.3) to the batch ingredients to act as a decolorizing agent. As can be seen from the working examples reported in the specification, however, the final crystalline articles most frequently display a residual trace coloration.
One essentially transparent glass-ceramic composition utilizing the decolorizing effect imparted by Nd.sub.2 O.sub.3 which exhibits the necessary melting and forming character for the commercial production of culinary ware, coupled with the chemical and physical properties required therein, demonstrates the following CIE chromaticity coordinates and transmission utilizing Illuminant C with ground and polished plates having a thickness of 6 mm.
x=0.3300.+-.0.0040 PA1 y=0.3360.+-.0.0040 PA1 Y=64.+-.4 PA1 Dominant Wave Length (.lambda.)=578 nm PA1 % saturation=10.5.+-.1 PA1 x=0.3300.+-.0.0040 PA1 y=0.3360.+-.0.0040 PA1 Y=60.8.+-.4 PA1 Dominant Wave Length (.lambda.)=578 nm PA1 % Saturation=10.5.+-.1
It is well-recognized that many types of culinary utensils, e.g., pots, casseroles, skillets, etc., require covers when used in certain kinds of cooking applications. For ease in monitoring the progress of cooking, a transparent cover has been deemed desirable. It is apparent that such a cover could be produced from a transparent glass-ceramic; perhaps the same glass-ceramic from which the utensil was produced. Because of the inherent greater cost of glass-ceramic materials, coupled with the fact that the cover customarily does not experience the severe mechanical and thermal shocks witnessed by the base ware, glass has been the predominant transparent material employed as covers for such utensils.
Accordingly, the primary objective of the present invention is to prepare glass compositions exhibiting the melting and forming capabilities necessary for shaping into covers for culinary utensils along with the chemical and physical properties intrinsically required in covers utilized for culinary ware. And most importantly, for aesthetic reasons, the glass should demonstrate a tint closely matching that of the glass-ceramic ware.